1. Field of the invention
The present invention relates to a position detector for a moving vehicle, and more particularly to a position detector for traveling a moving automobile, an unmanned mobile and carrying device in a factory, or a steering vehicle for use in agriculture, civil engineering machinery and the like along a predetermined traveling course.
2. Description of the Prior Art
Heretofore, as a system for detecting a present position of a moving body such as the moving vehicles as described above, there has been proposed a system provided with a means for scanning a light beam emitted from a moving body in the circumferential direction or every azimuthal direction centering around the moving body, light-reflecting means for reflecting to return the light beam in the direction of incident light and secured at at least three positions apart from the moving body, and a beam receiver means for receiving light reflected from the light-reflecting means (the Japanese Pat. Laid-open No. 67476/1984).
In the above mentioned prior art, differential azimuths between adjoining two of three light-reflecting means centering around the moving body are detected on the basis of received beam output of the light-receiving means, then a position of the moving body is operated based on the detected differential azimuths and positional information of the light-reflecting means which have been previously set.
In such a prior art system, if a moving body having a beam-receiving means mounted thereon is positioned in the neighborhood of the central portion of a triangle with the respective light-reflecting means secured at the three positions as the vertexes, the position of the moving body can be detected with a fairly high precision. However, there was a problem that, in a position apart from the neighborhood of the central portion of the triangle, it was difficult to obtain a high measuring precision.
As a countermeasure therefor, for instance, as described in U.S. patent application No. 413,934 (Japanese patent application No. 262191/1985) or the Japanese patent Laid-open No. 14114/1985 or No. 15508/1985, there have been proposed methods for measuring the position of a moving body wherein reference points are set on the vertexes of a quadrangle surrounding the area in which the moving body travels, optimum three reference points of the four reference points with which a high measuring precision can be expected are selected, and the positional detection of the moving body is performed on the basis of the positional information of the selected three reference points and the azimuths of the reference points viewed from the moving body.
Furthermore, U.S. patent application No. 413,935 (Japanese application Ser. No. 63-257912) discloses a system for detecting the position and advance direction of a moving body (hereinafter referred to as "moving vehicle") in which differential azimuths defined between two reference points disposed at diagonal positions among four reference points viewed from the moving vehicle are calculated on the basis of the azimuths defined by the four reference points with respect to the advance direction of the moving vehicle, and then the position and the advance direction of the moving vehicle are detected based on the azimuths defined by the four reference points, the differential azimuth defined between two reference points, and obtained positional information of the respective reference points.
In the above described system, however, there have been a case where a light beam cannot be projected on light-reflecting means disposed at the reference points because of tilt or vibration of the moving vehicle and/or a case where a light-receiving means installed on the moving vehicle receives light reflected from an object other than the above described light-reflecting means. If the reflected light of light beam cannot be positively received by the light-receiving means, a position of the moving vehicle is erroneously calculated, and as a result the moving vehicle cannot be allowed to travel along a predetermined course.
There is described in U.S. patent application No. 349,184 (US89A: Japanese patent application No. 63-262192) a detecting system which is so constructed that an azimuth of the reference point missed is so presumed in case of missing the reference points due to an inablility to receive reflected light, and a position of the moving vehicle is then calculated by using the azimuth angle of the presumed reference point.
In the above technique wherein a position of a moving vehicle is calculated by presuming the azimuth of a reference point, errors as a result of presumptions are accumulated with the increase of the missing number of times, so that accurate positional detection may not be attained. For this reason, it has been also proposed such a construction that, for example, travel of the moving vehicle is allowed to stop when the counted missing number of times exceeds a predetermined number of times. In this respect, however, most of the cases in which a reference point is missed are tentative phenomena. Accordingly, if a moving vehicle is allowed to frequently stop to interrupt its working because of such tentative phenomena, the working efficiency decreases.